In the field of drilling in the exploration for oil and gas, an important component is that of the formulation of drilling muds. Drilling muds are the fluids which are used to maintain pressure, cool drill bits and lift cuttings from the holes and vary in composition over a wide spectrum. Generally, drilling muds are based on aqueous formulations or oil-based formulations.
The drilling muds formed from these latices of the sulfonated thermoplastic terpolymers exhibit markedly improved low and high temperature rheological properties as compared to drilling muds formed from sulfonated thermoplastic copolymers.
A conventional oil-based drilling mud formulation is comprised of basically the following ingredients: oil (generally No. 2 diesel fuel), emulsifying agents (alkaline soaps and fatty acids), wetting agents (dodecylbenzene sulfonate), water, barite or barium sulfate, (weighting agent), asbestos (employed as viscosification agent) and/or, amine-treated clays (also as viscosification agent).
The above combination of ingredients is generally formulated to possess various weights based primarily on amount of barite added. For example, a typical drilling mud can vary in specific gravity from a range of about 7 pounds per gallon up to 17 pounds per gallon or even greater. This variation in specific gravity is primarily controlled by the amount of barite added. The above formulations perform adequately in a number of applications, primarily those where the use of oil-based drilling muds is dictated by the lack of stability of the formation in which drilling is taking place. For example, in various types of shale formation, the use of conventional water-based muds can result in a deterioration and collapse of the shale formulation. The use of the oil-based formulations circumvents this problem. However, it is observed that the current oil-based drilling muds have some significant disadvantages. One disadvantage is that the incorporation of asbestos or asbestos fines can incur significant health problems, both during the mud formulation and potentially during the subsequent use of such formulations. Therefore, it is desirable to eliminate the use of asbestos completely in such drilling muds. On the other hand, the use of substitutes for asbestos in this application has heretofore not been particularly successful in that the resulting viscosification agents must maintain adequate viscosities under the drilling conditions which can involve high temperature and high shear conditions.
There has been a substantial need for a drilling fluid which would exhibit good performance at low and high temperature in water sensitive formations. Past experience has shown that oil-based drilling fluids can provide good performance in water sensitive formations, and the state of the art systems can perform well up to about 350.degree. F. Typically, in such formations, the failure of the viscosities in current muds is circumvented by the addition of more viscosifier during the circulation of the drilling mud. While this solution is adequate at moderate temperatures, when much higher temperatures are encountered (example: geothermal wells or natural gas wells), the degradation of the viscosifier can be so rapid that the additional costs for a viscosifier can be uneconomical. There is a need, therefore, for drilling fluids which can maintain their viscosity and gel strength at temperatures up to and exceeding 400.degree. F. These needs are not adequately met by the current drilling fluids, even with the oil-based drilling muds often employed.
The instant invention will describe a family of viscosifiers suitable for oil-based drilling muds which offer thermal stability over a very broad temperature range, which appears to go far beyond that offered by state-of-the-art drilling muds. The types of viscosifiers employed currently in oil-based drilling muds typically are asbestos or amine coated clays. Such systems provide adequate gel strength (i.e., the ability to suspend solids) at temperatures up to 300.degree. F.
We have previously shown that lightly sulfonated polystyrene is very effective as a viscosifier for oil-based drilling muds. We have found that sulfopolystyrene provides good viscosification characteristics at temperatures exceeding 150.degree. F. when formulated in mud based on saturated salt water. Higher temperatures (300.degree., 400.degree. F.) encountered in accelerated aging studies for 15 hours have shown that sulfo-polystyrene enhances its efficacy at those higher temperatures. However, at low temperatures (&lt;150.degree. F.), sulfo-polystyrene was not an effective viscosifier. That is, the drilling mud containing this polymer needed to be "thermally activated" before acceptable rheological properties were attained. However, based on available information, it appeared that until that time, there was no viscosifier which was effective at temperatures around 400.degree. F. for the typical aging period of 16 hours. The instant invention will describe polymeric materials capable of providing excellent rheological properties, specially gel strength, at low and high temperatures (400.degree. F.). These materials may be effective at even higher temperatures.
The mechanism through which this new family of sulfonated terpolymer ionomers is operative is not completely understood. The preferred species of these materials is lightly sulfonated styrene: t-butylstyrene copolymers with a sulfonate level running at about 5 to about 100 milliequivalents of sulfonate groups per 100 grams of sulfonated terpolymer. It is believed that this polymer is marginally soluble in the hydrocarbon phase and completely insoluble in the aqueous phase of a formulated mud and, therefore, the polymer is probably located at the interface. As a consequence, this polymer is an extremely effective viscosifier even at quite low levels.
The instant invention discloses a class of sulfonated thermoplastic terpolymers which are effective viscosifiers at both room temperature and 400.degree. F. These sulfonated thermoplastic polymers are terpolymers of t-butyl styrene/styrene/sodium styrene sulfonate. These terpolymers provide excellent gel strengths at both room temperature and 400.degree. F.
A second facet of the instant invention relates to the use of these materials in formulations which employ high concentrations of salt in the aqueous phase. The sulfonated terpolymers of this invention do not lose some of their efficacy in salt water. It has been found that the use of a suitable non-ionic emulsifier with the sulfonated terpolymers is not required in the formulations to be are effective with salt water. Therefore, these terpolymers give formulations which perform well at high temperatures and in the presence of salt water phases, which is a highly desired objective in the drilling fluids industry.